This invention relates to improvements in stuffing boxes, and more particularly, to a novel conical packing gland assembly for utilization with polished rod stuffing boxes for oil and gas wells and the like.
Many producing wells today do not have sufficient subsurface pressure to lift fluids from the formation by a natural flow process. Thus, many of the wells are produced by means of reciprocating-type bottom hole pumps. The pump is usually actuated by a string of sucker rods which extend longitudinally downward through the tubing string into the well bore. The uppermost rod is called a polished rod and usually reciprocates within a stuffing box which is secured to the well head. A plurality of resilient packing gland members are disposed within the stuffing box around the polished rod for providing a seal between the box and the rod, thereby preventing pressurized borehole fluids from leaking around the rod and out the box which cause pollution problems and the like around the wellsite.
It was learned from field experience that packing glands having a generally conical shape were usually the most desirable in operation and accordingly received wide acceptance in the industry. Examples of such glands and their use in stuffing boxes may be found in U.S. Pat. Nos. 2,833,572 to Moseley, and 2,994,549; 3,002,776; 3,013,831; and 3,050,312, all to Tschappat. Nevertheless, several serious problems were associated with such glands as they were employed in the conventional stuffing box designs of the prior art.
Well head pressures at the stuffing box may, in some cases, be in the relatively low range of 40 pounds per square inch, which, nevertheless, would result in leakage out of the stuffing box due to normal wear about the seal, sometimes even after a relatively brief period of use.
However, several factors would frequently cause pressures to increase to as much as 1,000 or 2,000 psi, greatly increasing the problems encountered in effecting a good seal about the rod.
For example, as the search for subsurface petroleum reserves became more competitive, this lead to production from deeper wells wherein higher wellhead pressures were required to effect production.
Moreover, production in low temperatures results in oil of a relatively high viscosity also requiring greatly increased wellhead pressures for such production. These low temperature conditions may result from production during winter months or from production in more remote, colder climates, which is also becoming more commonplace as competition for petroleum reserves becomes more keen.
Still further, relatively high viscosities may also result from many crudes having a low API gravity (in the range of, say 8.degree.-18.degree.) even at normal temperatures, such as 70.degree. F., also requiring the aforementioned attendant increases in wellhead pressures necessary for production.
Yet another factor contributing to greatly increased pressures which must be handled by the stuffing box seals is the use of relatively long flow lines employed between the wellhead and the tank battery. Pressures due to these long lines may, of course, be increased even further during the aforementioned low temperature situations due to exposure of the flowline to the environment. These higher pressures, in turn, also contribute to the aforementioned leakage problems.
Several solutions to the hereinbefore noted problems were attempted. One such attempt was to provide for manual maintenance procedures for the conventional stuffing boxes whereby seals were periodically tightened about the rod. Not only was this procedure time consuming and costly, but it required tightening within a range which was often difficult to determine in the field--too much tightening resulted in overheating of packing glands and excessive wear on and damage to the glands, whereas if the seals were not tightened enough, leakage would soon reoccur. Yet another attempted solution when the leakage became excessive, was to "repack" the box, e.g., suspend production while seals were replaced in their entirety--again, a highly costly and undesirable procedure which, in extreme conditions, might even have to be done on a daily basis.
Still a further attempted solution was to provide for larger and more numerous seals, often resulting in stuffing boxes containing four large, thick packing glands. One serious difficulty with this approach was that modern petroleum production often results in concomitant production of highly corrosive substances such as hydrogen sulfide. Whereas exotic polymer materials were available capable of withstanding this harsh chemical environment from which packing glands could be fashioned, these materials are quite expensive. Accordingly, due to the volume of material required by this proposed solution to adequate effect a seal, less costly and thus less corrosion-resistant materials were utilized, resulting ultimately in return of the leakage problems.
Even if such corrosion-resistant materials were employed, yet an additional difficulty was encountered in providing larger, more numerous, and thus bulkier seals, namely the lack of space on the wellhead. Often stuffing boxes are installed on wells which have been in production for a long period. As technology has improved and environmental and safety concerns have come to the fore, many older wells are being upgraded with additional equipment, such as blowout preventers and pollution control devices. The total available cumulative height of all wellhead equipment including these devices, e.g., the "christmas tree", is often limited by the earlier positioning of the pumping platform. For this reason, the overall height or profile of a stuffing box is of critical concern to producers, with height differentials of even one inch dissuading or preventing a producer from utilizing an additional add-on to the christmas tree.
Therefore, it is a disadvantage of conventional stuffing boxes that, due to the aforementioned large multiple packing gland members and the large gland caps associated therewith to be hereinafter described, their height often exceeds the range tolerated by a particular platform.
Yet another problem with the prior art was that machining to close tolerances was required for the gland follower elements which provided downward sealing force against the glands to prevent damage to them. Still a further problem associated with the prior stuffing box designs was that the high pressures from the borehole had a tendency to open up the packing glands thereby contributing to the sealing problems.
One further problem with stuffing boxes of the prior art was associated with the fact that at times the polished rod, for various reasons, would become vertically misaligned with respect to the box, resulting in excessive wear and leakage about the packing glands. In the past, this required, in extreme cases, dismantling of the christmas tree and re-plumbing of the rod. It was thus highly desirable to provide a means for substantially reducing or eliminating the problems arising from this misalignment.
The disadvantages of the prior art are overcome by the present invention and an improved apparatus is provided for establishing a fluid-tight seal around a reciprocating polished rod in a stuffing box.